The present invention relates to a rapidly crosslinking liquid fluoropolymer containing the essential components tetrafluoroethylene and at least one compound from the group diiodomethane, 1,2-diiodo-1,1-difluoroethane, 1-iodo-2-bromo-1,1-difluoroethane, 1-bromo-2-iodo-1,1-difluoroethane and/or 1,2-dibromo-1,1-difluoroethane, which has certain portions on the terminal group of the formula xe2x80x94CH2xe2x80x94X where X=Br and/or iodine and iodine/bromine contents in the range 0.05 to 1 wt. % and a certain molecular weight and viscosity, a process for producing it and its use.
There is a general wish in the rubber industry for better processability of the rubbers used. This relates in particular to flow properties. The lower the viscosity of the rubber, the easier the processing technology and thus the greater the productivity and the smaller the amount of waste. These aspects are particularly relevant to fluororubbers, as they are expensive rubbers which cannot all be processed in the injection moulding machinery of the rubber industry.
Most fluororubbers with Mooney viscosities (ML1+10 at 120xc2x0 C.)  greater than 60 can be processed only by compression- or transfer moulding methods. Fluororubbers with Mooney viscosities of xe2x89xa660 can be processed in special injection moulding machines for solid rubbers, but this requires long cycle times and also produces a considerable amount of waste (flash-out).
Rubbers with Mooney viscosities (ML1+10 at 120xc2x0 C.) of 20-60 Mooney units are known, which can be processed into compression mouldings by this principle. The mechanical properties of these rubbers are not noticeably impaired [P. Ferrandez, St. Bowers, Gummi Fasem Kunstst. 48 (1995) 626-633].
A greater reduction of the molecular weight of rubbers, particularly fluororubbers, in order to reduce their viscosity still further impairs the properties of the vulcanised material, in particular the strength. Liquid fluororubbers are known from U.S. Pat. No. 5,852,125. However, the fluororubbers described here have a lower molar mass and a higher iodine content than those disclosed in this application. They are also slower to crosslink.
Furthermore, storage stability is still a problem today with many mixtures. The low-molecular fluororubbers with iodine contents of 1 to 30 wt. % described in U.S. Pat. No. 4361678, for example, which contain a molecular weight regulator of the type Rf/Ix, wherein Rf is a perfluorocarbon-, chloroperfluorocarbon- or chlorofluorocarbon group, are not stable during storage. The lack of storage stability is probably due to the fact that the iodine atom in the regulator is bonded to a carbon atom containing at least one fluorine atom, preferably to a chloroperfluorocarbon group, and thus at least half of the resulting terminal groups have the structure xe2x80x94Rfxe2x80x94I, e.g. xe2x80x94CF2xe2x80x94I. The iodine-carbon bond in such groups is particularly labile and iodine can very easily be split off thermally or under the influence of light. This makes the handling of fluoropolymers with high iodine contents very difficult. The reactive groups intended for crosslinking may also react prematurely and the resulting polymeric radicals may recombine. Crosslinked mouldings produced from this material also have poor ageing properties, see V. Arcella et al., Kautsch., Gummi, Kunstst., 44 (1991) 833-837.
According to WO 94/07929, regulators, which contain at least one iodine or bromine atom on a CH2 group, as described e.g. in JP-A 60 221 409 and EP-A 101 930, have the disadvantage that they severely retard polymerisation. Hitherto, they have been used only to produce fluororubbers with high molecular weights or low iodine contents.
Of the non-aqueous processes, polymerisations in the pure liquefied fluoromonomer have proved disadvantageous, as most of the resulting polymers are not soluble in it and also swell only a little. A reproducible polymerisation with good heat and mass transfer and thus acceptable space-time yields is equally impossible by this means.
However, fluoromonomers can be polymerised well in the presence of certain fluorine-containing solvents, see e.g. U.S. Pat. No. 4,243,770, DE-A 196 40 972.1. U.S. Pat. No. 5,182,342 describes the use of fluorocarbons in the presence of up to 20% water as polymerising medium, which fulfill certain criteria with regard to the F/H ratio and the position of the hydrogens. With all compounds of this type, which contain hydrogen and optionally also chlorine, there is always the problem that they may enter into these transfer and/or termination reactions.
In WO-98/15583, 1,1,2-trichlorotrifluoroethane is used as the polymerising medium. However, compounds of this type (chlorofluorocarbons) have considerable ozone-damaging potential and for this reason, their use in industry is prohibited in many industrialised countries. The fluororubbers described in this patent contain 0.5-2.5 wt. % iodine terminal groups.
In the application DE 197 40 633, liquid fluororubbers are produced in inert solvents of the type RF-SO2F or perfluoroalkylsulfone in the presence of a molar mass regulator. The fluororubbers described here also have iodine or bromine terminal groups, but the combination of features in this application and their surprising influence on the crosslinking speed is not mentioned.
Application WO-A-98/15583 describes a liquid fluororubber and a process for producing it. However the combination of features in this application and their surprising influence on the crosslinking speed is not mentioned and the molar masses are too small.
There was thus still a need for liquid fluororubbers, which are pumpable at least at slightly raised temperatures (60-120xc2x0 C.) and can be processed in conventional thermoplastic processing machines. These liquid fluororubbers should also remain stable during storage and crosslink rapidly and the resulting rubber pieces should have good mechanical and ageing properties, which are very close to those of conventional solid fluororubbers.
The object of the present invention was therefore to provide fluororubbers which have these properties.
A further object of the invention was to remove, at least partially, the disadvantages of the fluororubbers known from the prior art.
It was found, that fluororubber consisting of the essential components tetrafluoroethylene and at least one compound from the group diiodomethane, 1,2-diiodo-1,1-difluoroethane, 1-iodo-2-bromo-1,1-difluoroethane, 1-bromo-2-iodo-1,1-difluoroethane and/or 1,2-dibromo-1,1-difluoroethane,
which has certain portions on the terminal group of the formula xe2x80x94CH2xe2x80x94X where X=Br and/or iodine and iodine and/or bromine contents in the range 0.05 to 1 wt. % and a certain molecular weight and viscosity, solves this problem.
The present invention therefore relates to a liquid fluoropolymer which can be produced from tetrafluoroethylene and optionally other fluorine-containing and/or non-fluorine containing monomers and at least one co-compound from the group diiodomethane, 1,2-diiodo-1,1-difluoroethane, 1-iodo-2-bromo-1,1-difluoroethane, 1-bromo-2-iodo-1,1-difluoroethane and/or 1,2-dibromo-1,1-difluoroethane, in which at least 80% of the terminal groups has the formula xe2x80x94CH2xe2x80x94I and/or xe2x80x94CH2xe2x80x94Br and which has a complex viscosity at 100xc2x0 C. and xcfx89=6.3 sxe2x88x921 of 0.01-30 kpas and a temperature index, calculated as the quotients of the viscosities at 40 and 100xc2x0 C. of 3-250, characterised in that, the fluoropolymer
a) has more than 10 mol % repeated units derived from tetrafluoroethylene,
b) contains in the range of 0.05 to 1 wt. % iodine and/or bromine and
c) has an average molecular weight (number average Mn) of over 25,000 g/mol.
Fluorine-containing monomers according to the invention are preferably fluorinated, optionally substituted ethylenes, which may contain, besides fluorine, hydrogen and/or chlorine, such as e.g. vinylidene fluoride and chlorotrifluoroethylene, fluorinated 1-alkenes with 2-8 carbon atoms, such as e.g. hexafluoropropene, 3,3,3-trifluoropropene, chloropentafluoropropene, pentafluoropropene, hexafluoroisobutene and/or perfluorinated vinyl ethers of the formula CF2xe2x95x90CFxe2x80x94Oxe2x80x94X where X=C1-C3 perfluoroalkyl or xe2x80x94(CF2xe2x80x94CFYxe2x80x94O)nxe2x80x94RF, wherein n=1-4, Y=F or CF3 and RF=C1-C3 perfluoroalkyl.
Non-fluorine-containing monomers according to the invention are preferably ethylene, propene, isobutene or vinyl esters, such as e.g. vinyl acetate.
The fluoropolymer according to the invention is preferably a (co)polymer obtained by (co)polymerisation of a mixture of tetrafluoroethylene, vinylidene fluoride and other fluorine-containing and/or non-fluorine-containing monomers.
The combination of tetrafluoroethylene, vinylidene fluoride, hexafluoropropene and optionally perfluorinated vinyl ethers such as e.g. perfluoro-(methyl-vinyl-ether) is preferred in particular.
The fluoropolymer according to the invention contains more than 10 mol % units derived from tetrafluoroethylene and preferably less than 60 mol % units derived from vinylidene fluoride, and also optionally other fluorine-containing or non-fluorine-containing monomers.
Derived means, that the compounds concerned are used as monomers.
The co-compound is preferably diiodomethane.
The co-compounds can easily be obtained e.g. by adding halogen or interhalogen to vinylidene fluoride.
More than 90% of the terminal groups preferably have the formula xe2x80x94CH2X, where X=iodine or bromine. The iodine and/or bromine content is in the range 0.1 to 1 wt. %, preferably 0.5-1. Iodine is preferred in particular. The iodine and/or bromine is positioned only at the end of the chain.
As was demonstrated by 19Fxe2x80x94 and 1H NMR spectroscopy, the fluoropolymer according to the invention contains no groups of the formula xe2x80x94CF2xe2x80x94I or  greater than CFxe2x80x94I, from which iodine could be split off particularly easily either thermally or by the influence of light.
The number average of the molecular weights Mn is preferably higher than 25,000 g/mol, in particular higher than 30 000, but always provided that the fluoropolymer has a complex viscosity at 100xc2x0 C. and xcfx89=6.3 sxe2x88x921 of 0.01 to 30 kpas and a temperature index, calculated as the quotients of the viscosities at 40 and 100xc2x0 C., of 3 to 250.
In a preferred embodiment of the invention, fillers such as e.g. carbon black, silica, TiO2 (mixture containing filler) and/or crosslinking chemicals, i.e. catalysts and co-crosslinkers (crosslinkable mixtures optionally containing filler), such as e.g. an organic peroxide and a triallylisocyanurate (see e.g. EP-A 398 241) or bisamines/bisphenols in combination with phase transfer catalysts and metal oxides as described in A.L. Logothetis, Prog. Polym. Sci., Vol. 14, (1989), 251-296, are added to the fluoropolymer according to the invention.
The fluoropolymers according to the invention or mixtures containing filler produced from them still have a consistency at room temperature similar to that known for solid rubbers, i.e., higher viscosity, the viscosity falling sharply as the temperature rises. At temperatures of 60xc2x0 C. to 120xc2x0 C. they have a quasi-liquid consistency. The complex viscosities, measured in a Scher oscillation experiment with a Bohlin rheometer of the type VOR-Melt (cycle frequency xcfx89=6.3 sxe2x88x921), given in kPa s preferably fall into the following characteristic ranges:
Accordingly, the temperature index, calculated as the quotient of the viscosities at 40xc2x0 C. and 100xc2x0 C., is preferably 3 to 250 for the fluoropolymer, or 3-300 for the fluoropolymer containing filler (mixture containing filler).
In one embodiment of the invention, the fluoropolymer according to the invention is crosslinked to form rubbery-elastic mouldings. This is preferably done radically, by radiation or using peroxides. The mixtures containing filler and also the crosslinkable mixtures optionally containing filler, can preferably be crosslinked by radiation or using peroxides to form rubbery-elastic mouldings.
The invention also relates to a process for the production of the fluoropolymer according to the invention, in which tetrafluoroethylene is radically polymerised, optionally with another fluorine-containing or non-fluorine-containing monomer in the presence of at least one co-compound containing iodine and/or bromine from the group diiodomethane, 1,2-diiodo- 1,1-difluoroethane, 1-iodo-2-bromo-1,1 -difluoroethane, 1-bromo-2-iodo-1,1-difluoroethane and/or 1,2-dibromo-1,1 -difluoroethane at temperatures of xe2x88x9220xc2x0 C. to +130xc2x0 C., preferably +20xc2x0 C. to +100xc2x0 C., in the presence of an initiator and/or other auxiliary substances, preferably a redox initiator system.
Polymerisation can be carried out in solution, suspension or emulsion. Polymerisation in aqueous emulsion in the presence of a redox initiator is preferred.
The quantity of co-compound is in the range 0.1 to 1 wt. %, preferably 0.5-1 wt. %, in relation to the fluoromonomers to be reacted.
A solvent with a low transfer constant such as e.g. hexafluorocyclopentane, perfluorohexane, perfluoro(tributylamine) or trichlorotrifluoroethane, can be used as the medium for solution polymerisation.
However a solvent without ozone-damaging potential, such as certain carbon fluoride compounds or fluorocarbon compounds containing fluorocarbon atoms or heteroatoms, such as 1,1,1,3,3-pentafluoropropane, 1,1,1,2,3,3-hexafluoropropane, 1,1,2,2,3,3-hexafluorocyclopentane, 1,1,2,2-tetrafluorocyclobutane, 1-trifluoromethyl-1,2,2-trifluorocyclobutane, 2,3-dihydrodecafluoropentane, 2,2-bis(trifluoromethyl)-1,3-dioxolane, perfluoro(tripropylamine), methoxy-2-hydrohexafluoropropane, methoxynonafluorobutane, perfluorobutane sulfofluoride, perfluorosulfolane and also the compounds of the formula (I) or (II) mentioned in the previous application DE-197 40 633.5, 
wherein R1 is a fluoroatom or a perfluoroalkyl group containing 1-4 C atoms and R2 is a perfluoroalkyl group containing 1-4 C atoms and n=4 or 5, in particular perfluorobutane sulfofluoride and perfluorosulfolane is preferred for solution polymerisation.
1,1,1,3,3-pentafluoropropane, perfluorobutanesulfofluoride and perfluorosulfolane, alone or in mixture, are preferred.
It is advantageous for the solvents to have low boiling points in order to facilitate separation of the solvent and the fluoropolymer after polymerisation. Because of their low boiling points of 15 to 70xc2x0 C. and their low enthalpy of evaporation, the preferred compounds mentioned can easily be separated from the rubber after polymerisation by distillation.
The ratio of fluoromonomer (monomer) to solvent and the reactor fill level is preferably chosen in such a way that the proportion of monomer is at least 20 wt. % of the liquid phase at the reaction temperature. The quantity of monomer dissolved in the liquid phase can be determined e.g. from the mass balance on the basis of the partial pressures of the monomer in the gas phase.
Organic or, optionally, fluorinated peroxides such as e.g. tert.-butyl perpivalate, diisopropyl peroxidicarbonate or trifluoroacetyl peroxide or azo compounds such as azo-bis-(isobutyronitrile) or azo-bis(2,4-dimethylvaleronitrile) which are soluble in the monomer or solvent are preferably used as the initiator.
For aqueous emulsion polymerisation, fluorinated emulsifiers, such as for example the salts of C6-C12 perfluorocarboxylic or sulfonic acids which are soluble in water, are used in concentrations of 0.05 to 2 wt. % as auxiliary substances to stabilise dispersion. Examples of these are sodium- or ammonium salts of perfluorooctanoic acid and the lithium salt of perfluorooctylsulfonic acid.
Inorganic peroxides, such as e.g. peroxidisulfates, perborates, percarbonates, generally in the form of their potassium, sodium or ammonium salts can also be used as initiators, preferably in combination with reducing agents. The following can be used as reducing agents: sulfur compounds, such as sodium sulfite, sodium pyrosulfite or Rongalit C (sodium formamidine sulfinic acid), other organic reducing agents, such as ascorbic acid, metal salts, such as iron-(II)- or cobalt-(II)-salts, metalorganic compounds etc. A system consisting of at least one manganese compound in oxidation stages xe2x89xa73 and optionally a reducing agent, such as e.g. carboxylic acids, dicarboxylic acids, polyvalent alcohols and hydroxycarboxylic acids is preferred as the redox initiator system.
The fluoropolymers according to the invention can be produced in batches, but preferably by semi-continuous or continuous processes.
Production is carried out under autogenic pressure, which is set depending on the reactor fill level, temperature and monomer quantities.
The invention also relates to the use of the fluoropolymers according to the invention for the production of coatings or for polymer-analogous reactions for the substitution of iodine or bromine groups preferably by other reactive groups. The mixtures containing filler, or crosslinkable mixtures optionally containing filler, can be used for the production of coatings or for polymer-analogous reaction for the substitution of iodine- or bromine groups by other reactive groups by means of nucleophilic or radical substitution.
The iodine- and bromine groups are preferably substituted by reaction with compounds, which contain primary or secondary amino groups, allyl- or vinyl groups and alkylatable aromatic groups.
The invention also relates to a process for the production of fluoro-elastomeric mouldings and/or coatings, according to which fluoropolymers with a viscosity-temperature index at 40/100xc2x0 C. of 3 to 250, at temperatures of 40 to 250xc2x0 C. under pressure, preferably 20 to 200 bar, are injection moulded in moulds heated to 100 to 250xc2x0 C., preferably 120 to 200xc2x0 C. in which they are then crosslinked.
Injection moulding is carried out in commercial injection moulding- or piston-type metering machines as described e.g. in Comprehensive Polymer Science, Vol. 7 (1989) p. 356. The fluoropolymer is preferably fed into the thermoplastic injection moulding- or piston-type metering machine heated to a temperature of 40 to 250xc2x0 C. Heated metering and feeding machines are particularly suitable for this, preferably heated feeder piston pumps.
The fluoropolymers according to the invention or a mixture of several fluoropolymers according to the invention, optionally in the presence of other liquid rubbers are preferably used as fluoropolymers.
Mixtures containing filler or crosslinkable mixtures optionally containing filler, can also be used.
The improved flowability makes it possible to use crosslinking chemicals or chemicals which activate/initiate crosslinking, such as e.g. peroxides, bases or initiators with increased reaction speeds. These produce a crosslinking curve in the rheometer at half-times for 50% increase of the shear modulus, called t50, of 1 to 20 minutes at 100xc2x0 C.
The fluoropolymers can be crosslinked in the moulds heated to 100 to 250xc2x0 C. e.g. directly via the terminal iodine or bromine atoms as reactive groups. This may be done by the usual radical method using substances which form radicals, such as organic peroxides or by nucleophilic substitution of the terminal iodine, for example by means of a polyfunctional amines.
To produce the mouldings, the fluoropolymer mixtures are injection moulded at temperatures of 40 to 250xc2x0 C. under pressure, preferably 20 to 400 bar, in moulds heated to 100 to 250xc2x0 C., preferably 120 to 220xc2x0 C., in which they are then radically crosslinked. Injection moulding is carried out in commercial thermoplastic injection moulding- or piston-type metering machines.
Furthermore, the terminal iodine atoms can also be replaced by other reactive groups and then crosslinked. Nucleophilic substitution reactions for example, such as amination or saponification can be used for this. Terminal Cxe2x80x94C double bonds can be introduced e.g. by radical addition, and subsequent elimination, of allyl acetate.
The fluoropolymer according to the invention is unusual in that it can be used with the advantageous technology of liquid rubber processing to produce crosslinked mouldings or coatings and forms networks at a surprisingly rapid crosslinking speed.
Hitherto, it has only been possible to use the combination of good flowability and extraordinarily high crosslinking speeds for covalently cross-linked elastomers to reduce production cycle times when producing elastomeric mouldings from liquid silicon rubbers.
The following examples explain the invention but are not intended to restrict it.
Embodiments: